1. Technical Field
The present invention relates to a two-constituent photo curable ink composition set that is mixed and then photoset by irradiation with light, and to a recording method and records employing the ink composition set.
2. Related Art
Inkjet recording is a printing method in which printing is brought about when droplets of an ink composition are propelled so as to adhere to a recording medium such as paper. A characteristic of inkjet recording is that images of high resolution and high quality can be rapidly printed.
Ink compositions used in inkjet recording are generally based on aqueous solvents, which contain color materials and wetting agents such as glycerin to prevent clogging.
On the other hand, when printing is done on recording media such as various types of paper and fabric that are not readily penetrated by ink compositions, or metal, plastic, or other materials that are not penetrated at all, such as sheets and films made from phenol, melamine, vinyl chloride, acrylic, polycarbonate, or other resins, the ink composition must include a component that allows the color material to be affixed to the recording medium in a stable manner.
Meanwhile, “inkjet recording media including at least a colorant and a component that is polymerized upon the provision of UV rays” have been proposed in the related art (see, for example, JP-A-3-216379). This inkjet ink recording medium is said to ensure high quality printing with better images on ordinary paper without compromising the many advantages of inkjet printing.
“Photo curable types of inkjet inks comprising color materials, photo curable agents (radical polymerizable compounds), (radical photo) polymerization initiators, and the like” have also been proposed (see, for example, U.S. Pat. No. 5,623,001). This ink is said to avoid the problem of the ink bleeding into the paper, allowing better quality to be achieved.
On the other hand, “an ink for an inkjet printer having the characteristic of being set by the irradiation of UV rays, wherein the ink for the inkjet printer comprises at least water, a water-soluble photo curable type of resin prepolymer with at least 2 functional groups in the monomer structure, a water-soluble or water-insoluble photoinitiator, and a water-soluble dye” has been proposed (see, for example, JP-A-5-186725). This ink is said to allow the following inks “1 through 6” to be provided.
1. It is possible to provide an ink that includes a dye as the color material in an aqueous system for coloring printing, yet has the effect of preventing the dye from bleeding even if moisture (ink in printing of second color or later) is again deposited after printing.
2. It is possible to provide an ink that does not readily dry out in head nozzles, even without the addition of agents to prevent drying, by using a UV-setting type of resin prepolymer.
3. It is possible to provide a printing mechanism in which, when using ink that contains a photo curable resin, the resin component and reaction initiator are stored separately to enhance the ink storage stability, and are sprayed from separate head nozzles to form print dots.
4. It is possible to provide an ink making use of a resin to which dye molecules are readily adsorbed so that, in inks that contain a photo curable resin, the resin component and dye component will not separate when the resin component is set.
5. It is possible to provide an ink that includes a photo curable resin, wherein the color material dye will not fade during UV irradiation.
6. It is possible to provide an ink capable of preventing pigment from drying out and aggregating in head nozzles, particularly when organic pigments are used as the color material in common aqueous-based inks for inkjet printers.
“A separable inkjet recording ink which is composed of an ink (A) including a monomer that becomes set upon the provision of energy and a solvent for dissolving the monomer, and an ink (B) including at least one pigment dispersion as a color material component and a solvent for the dispersion thereof” has also been proposed (see, for example, JP-A-8-218018). This ink is said to provide an inkjet recording ink that in itself has good storage stability and permits recording with good photo-resistance, heat-resistance, or print quality.
“A UV-setting ink including a photopolymerizable monomer, adhesive, UV sensitizer, and 20 to 40 wt % volatile organic solvent” has also been proposed (see, for example, JP-A-2004-18716). This UV-setting ink composition includes a volatile organic solvent, and the UV sensitizer substantially acts as a photoinitiator.
“A UV-setting white ink composition for inkjet recording, which includes a UV-curable compound, low-boiling solvent, photoinitiator, and sensitizer” has also been proposed (see, for example, JP-A-2005-298757). The low-boiling solvent is an essential constituent of this white ink composition, and the sensitizer is an amine that substantially acts as a polymerization promoter, which is added to minimize the inhibition of polymerization by oxygen during the radical polymerization. The inclusion of the volatile organic solvent is an advantage in that the viscosity of the ink composition can be lowered, but is also undesirable because of need for a drying treatment, as well as heat energy and the extra burden in terms of equipment, since the failure to remove the solvent prior to the setting reaction will result in printing defects caused by bubbles in the organic solvent, lower adhesion on the recording medium, and problems caused by the volatilization of the solvent over the long term. Since, furthermore, the release of organic solvents into the atmosphere is also environmentally undesirable, another major drawback is the need to set up recovery equipment or exhaust gas combustion equipment, making it impossible to consider this a very desirable option.
Other problems, in addition to the above, which occur during the use of the above ink compositions proposed in the past are that the sensitivity and photo curing rate are inadequate, and that the polymerizable compounds and polymerization initiators included in the ink compositions accelerating the curing reaction of the ink during storage.
In addition, when semiconductor elements, LED, LD, or the like are used as UV sources for the photo curing reaction, these elements ensure the efficient radiation of violet/blue visible light or longer wavelengths (350 nm to 400 nm) in the ultraviolet region with relatively low energy consumption, but a problem is that photoinitiators that produce radicals at these wavelengths tend to cause the set film to become yellow.
On the other hand, low-yellowing radical polymerization photoinitiators which do not cause set films to become yellow can efficiently produce radicals under UV rays with an absorption wavelength of 350 nm or less.
However, low pressure mercury lamps, high pressure mercury lamps, ultrahigh pressure mercury lamps, halogen lamps, xenon lamps, and the like are used to obtain light in this wavelength region, and a number of problems resulting from their use must be addressed, such as thermal damage to the recording media, and the need for larger-scale equipment, high power consumption, and incidental equipment (refrigeration, ventilation, shielding).